This invention is directed to reducing the energy required to improve the properties of multicomponent polymer systems which form pressure-sensitive adhesives.
More particularly, the invention is related to the use of halogenated crosslinking agents for radiation (EB & UV) curing of adhesives containing unsaturated elastomeric polymers, such as unsaturated block copolymers, especially, though not limited to, styrene-isoprene block copolymers, e.g., linear SI, SIS, multi-armed (SI).sub.x. Halogenated materials significantly enhance the facility of free-radical materials crosslinking of pressure-sensitive adhesive formulations containing unsaturated elastomers, leading, at lower levels of free-radical generating energy, to a higher degree of crosslinking, which is manifested by the formation of more insoluble gel, lower swelling ratios, and significant improvements in adhesive performance at elevated temperatures.
Conventional crosslinking systems for such adhesives have been based upon the use of polyfunctional acrylates or methacrylates. Polythiols have also been effectively used. These materials exhibit a number of disadvantages: for example; curing may cause a considerable loss of peel and tack, and the thermal instability of the crosslinkers requires in-line static mixing immediately prior to coating for hot melt processes. Many of the halogenated materials, in contrast, are thermally stable and are ideally suited to hot melt (100% solids) processing, although they may also be used in solution or emulsion processes.
Properties of unsaturated pressure sensitive adhesive compositions can be improved by use of actinic radiation, such as ultraviolet (UV) radiation; electron beam (EB) radiation. Normally, the adhesive is applied to facestock and/or a release liner and subjected to a suitable curing action to improve such properties as elevated temperature shear.
One means of cure is electron beam (EB) radiation. While the facestock and/or release liner can sustain electron beam dosages up to a certain level, e.g., 80 to 100 kiloGray (kGy), going beyond that level can result in degradation of components of adhesive label and tape constructions such as the face stock and/or release liner and/or adverse reactions between the adhesive and the silicon release agent of the release liner.
Increased radiation requirements, whether EB or actinic, will reduce the speed at which an adhesive coated substrate can pass under the radiating surface or increase the number of radiating surfaces. Both are costly.
The present invention is directed to reducing the energy required to achieve a positive modification in a pressure sensitive adhesive formed of at least two components, one an unsaturated elastomeric polymer component, the other an organic additive component which is one or more organic additives which are at least dispersable in the elastomeric component. Typically, the organic additive component is provided to tackify or plasticize the elastomeric component.